There is no question that sidewalk sheds can help in some situations. These are commonly used in New York to protect people while walking on the sidewalk if work is being done above them. The idea is that the sheds will keep falling debris from striking pedestrians and causing injuries. The problem is that a lot of these sheds are in such bad shape that they don't really work well at all.
One man recounted walking down a heavily-trafficked sidewalk with a shed built over it, and a piece of debris fell from the building. It was not stopped by the shed, and it slammed into his skull. It knocked him out. After he came to and went to get medical treatment, he needed numerous stitches to close up the wound. As a result, he started a lawsuit.